Processed meat products, such as poultry and ham, are often packaged in a flexible, thermoplastic, heat-shrinkable film tubing commonly referred to as a casing. Although some casings have a lay-flat width of 6-20 inches, some products, such as ham, etc., are quite often packaged in a casing of smaller lay-flat width, e.g., a width of from about 3 to 6 inches. Such casings often may need to have a precisely-controlled width, because the packages are stated as having a given weight, which is uniform among packages, and the packages also have product sliced at uniform intervals, with each package containing the same number of slices. Thus, variations in casing width can result in both an undesirable degree of variation in overall package weight, as well as an undesirable degree of variation in slice weights.
Thus, there is a need for a casing having a small and uniform diameter. However, it is relatively difficult to make a narrow width, heat-shrinkable seamless casing having a precisely controlled width, using a commercially feasible process. Consequently, there is a need for some other process for making a narrow-width, precisely-width-controlled casing.
Some backseamed casings are known to be casings of small and uniform diameter. Small-diameter backseamed casings are known which have a precisely controlled casing width, i.e., a lay-flat width independent of film extrusion variations. In the production of backseamed casings (e.g., using a backseaming machine such as a Nishibe HSP-250-SA backseaming machine obtained from Nishibe Kikai Co. Ltd. of Nagoya, Japan), a flat sheet of film is folded longitudinally by passage over a "forming shoe". A forming shoe is a part of the backseaming machine which the film is passed under and around, i.e., so that the initially flat film is reconfigured as a tube, having a longitudinal overlap and seal therealong (lap-sealed backseamed casing), or with film longitudinal edges abutted against one another (butt-sealed backseamed casing), with the width of the tube being determined by the circumference of the forming shoe. A longitudinal lap or butt seal is then applied while the film is between the forming shoe and a sealing device, resulting in a lap-sealed backseamed casing, or a butt-sealed backseamed casing. Butt-seal casings utilize a butt-seal tape sealed to the inside or the outside surface of the casing film, along both sides of the abutting longitudinal seam of the casing film. In either event, the resulting tubing, termed a "backseamed casing," is sealed or clipped at its ends after being filled with a meat product. For some uses, the meat product is thereafter cooked while in the backseamed casing.
It would be desirable to provide a highly uniform, small diameter backseamed casing suitable for cook-in end use, the casing being made from a film which adheres to high-protein meat products, such as certain grades of ham and turkey. Of course, it would also be desirable to provide the backseamed casing with a backseam seal which survives the cook-in process.
It is known that a polar surface is needed for adhesion of a film to a meat product. Adhesion of the film to the meat is frequently needed in order to prevent "purge", i.e., "cook-out", which can occur during the cooking of the meat packaged in the film if the film does adhere to the meat during cook-in. A polar film surface can be provided by using: (a) polar resin in the film layer in contact with the meat, and/or (b) surface modification, such as corona treatment, of the film surface in contact with the meat. Typically, polar polymers used for meat adhesion include: ethylene/unsaturated acid copolymer, anhydride-containing polyolefin, and polyamide.
Film-to-meat adhesion is known to be enhanced by corona treatment of the surface of the film to which the meat is to be adhered. However, corona treatment alters the film surface in a manner which can, on occasion, result in an inferior seal, i.e., a seal more likely to leak than if the film surface is not corona treated. This "leaky seal problem" can be avoided by "buffing off" the corona treatment in the area of the seal, so that the advantageous effects of the corona treatment, i.e., greater meat adhesion, can be retained on the majority of the meat-contact surface of the film, while at the same time avoiding, in the area of the seal, the seal-quality problem caused by the corona treatment. However, the buffing step is undesirable, as it is an additional processing step which renders the casing manufacture more complex and costly. Furthermore, the buffing step is frequently inconsistent.
Since the backseaming process is generally carried out after the corona treatment, shrinkage of the film against the forming shoe (during backseaming), coupled with forwarding the film over the forming shoe after shrinkage, results in the rubbing of the film against the forming shoe edges. This rubbing reduces or destroys corona treatment, at least in the area in which the film rubs against the forming shoe. As a result, backseamed casings containing corona treated films can exhibit purge at the locations at which the film rubs against the forming shoe. Furthermore, corona treatment can be inconsistent, at least with respect to prevention of purge for products having an intermediate protein content. It would be desirable that the casing film has a consistent and adequate level of protein/meat adhesion. As a result, it would be desirable to provide a corona-treatment-free backseamed casing which prevents purge from products relatively high in protein, where the adhesion of the casing film to the meat product is uniform over the film.
Thus, it would be desirable to provide a backseamed casing of small and uniform diameter which is heat-shrinkable and suitable for cook-in end use, exhibits good purge-resistance and good seal strength, can be economically manufactured, does not produce significant meat pull-off upon being stripped from a cooked meat product, and which provides a good oxygen barrier, in order to provide good shelf life to the cooked meat product.
Heat-shrinkable films having an outer layer capable of providing meat adhesion, which are otherwise suitable for use as backseamed casings, have been found to have the undesirable characteristic of necking down on the forming shoe during the backseaming process. The necking down on the forming shoe is believed to be due to shrinkage of the film during the heat sealing step of the backseaming operation. That is, the heat sealing step can cause substantial film shrinkage in an area extending outward from the seal, causing the edges of the casing to neck down on the forming shoe. The result of necking down is a casing having "ruffled edges", i.e., visible nonuniformities in the casing. In an extreme case, necking down results in the rupture of the film, as the shrinking of the film against the forming shoe places so much force on the film that the film ruptures. Thus, it would be desirable to provide a casing film which does not shrink down (i.e., "neck down") on the forming shoe during the backseaming operation.